Action Potential

• Depolarization– Initially, this is a local electrical event called

end plate potential

– Later, it ignites an action potential that spreads in all directions across the sarcolemma

Action Potential

• Polarized Sarcolemma– Initially, this is a local

electrical event called

end plate potential– Later, it ignites an

action potential that

spreads in all directions

across the sarcolemma

Action Potential

• Polarized Sarcolemma– The predominant extracellular ion is Na+

– The predominant intracellular ion is K+

– The sarcolemma is relatively impermeable to both ions

Action Potential

• Depolarization and initiation of action potential.– An axonal terminal of

a motor neuron releases

ACh and causes a patch

of the sarcolemma to

become permeable to

Na+ (sodium channels open)

Action Potential

• Depolarization and initiation of action potential.– Na+ enters the cell,

and the resting potential

is decreased (depolarization

occurs)– If the stimulus is strong enough, an action

potential is initiated

Action Potential

• Propagation of Action Potential– Polarity reversal of the

initial patch of sarcolemma

changes the permeability

of the adjacent patch– Voltage-regulated Na+

channels now open in

the adjacent patch causing

Action Potential

• Propagation of Action Potential– Thus, the action potential

travels rapidly along the sarcolemma

– Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

Action Potential• Repolarization

– Immediately after the depolarization wave passes, the sarcolemma permeability changes

– Na+ channels close and K+ channels open

– K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma

Action Potential• Repolarization

– Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)

– The ionic concentration of the resting state is restored by the+-K+ pump

Action Potential• Excitation-Contraction Coupling

– Once generated, the action potential:• Is propagated along the sarcolemma• Travels down the T tubules• Triggers Ca2+ release from terminal cisternae

– Ca2+ binds to troponin and causes: • The blocking action of tropomyosin to cease• Actin active binding sites to be exposed

Action Potential• Excitation-Contraction Coupling

– Myosin cross bridges alternately attach and detach

– Thin filaments move toward the center of the sarcomere

– Hydrolysis of ATP powers this cycling process– Ca2+ is removed into the SR, tropomyosin

blockage is restored, and the muscle fiber relaxes

Action Potential• Excitation-Contraction Coupling

Action Potential

• Role of Ca+2

– At low intracellular Ca2+ concentration:

• Tropomyosin blocks the binding sites on actin

• Myosin cross bridges cannot attach to binding sites on actin

• The relaxed state of the muscle is enforced

Action Potential

• Role of Ca+2

– At higher intracellular

Ca2+ concentrations:• Additional calcium binds

to troponin (inactive

troponin binds two Ca2+)• Calcium-activated troponin

binds an additional two Ca2+

at a separate regulatory site

Action Potential

• Role of Ca+2

– Calcium-activated troponin

undergoes a conformational

change– This change moves

tropomyosin away from

actin’s binding sites

Action Potential

• Role of Ca+2

– Myosin head can now

bind and cycle– This permits contraction

(sliding of the thin filaments

by the myosin cross bridges)

to begin

Sequence of Events in Contraction

• Cross bridge formation – myosin cross bridge attaches to actin filament

• Working (power) stroke – myosin head pivots and pulls actin filament toward M line

• Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches

• “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state

Sequence of Events in Contraction

Myosin head (high-energy configuration)

ADP and Pi (inorganic phosphate) released

Myosin head (low-energy configuration)

Contraction of Skeletal M.

• Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)

• Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening

• Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

Contraction of Skeletal M.

• Contraction of muscle fibers (cells) and muscles (organs) is similar

• The two types of muscle contractions are:– Isometric contraction – increasing muscle

tension (muscle does not shorten during contraction)

– Isotonic contraction – decreasing muscle length (muscle shortens during contraction)

Contraction of Skeletal M.

• A motor unit is a motor neuron and all the muscle fibers it supplies

• The number of muscle fibers per motor unit can vary from four to several hundred

• Muscles that control fine movements (fingers, eyes) have small motor units

Contraction of Skeletal M.

Contraction of Skeletal M.

• Large weight-bearing muscles (thighs, hips) have large motor units

• Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Types of Contractions

• A muscle twitch is the response of a muscle to a single, brief threshold stimulus

• The three phases of a muscle twitch are:– Latent period –

first few milli-seconds after stimulation when excitation-contraction coupling is taking place

Types of Contractions

(Twitch)– Period of contraction – cross bridges actively

form and the muscle shortens– Period of relaxation –

Ca2+ is reabsorbed into the SR, and muscle tension goes to zero

Muscle Response

• A single stimulus results in a single contractile response – a muscle twitch

• Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation

Muscle Response

• More rapidly delivered stimuli result in incomplete tetanus

• If stimuli are given quickly enough, complete tetanus results

Muscle Response

• Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs

• Beyond threshold, muscle contracts more vigorously as stimulus strength is increased

• Force of contraction is precisely controlled by multiple motor unit summation

• This phenomenon, called recruitment, brings more and more muscle fibers into play

Stimulus Intensity and Muscle Tension

Treppe

• Staircase – increased contraction in response to multiple stimuli of the same strength

• Contractions increase because:– There is increasing availability of Ca2+ in the

sarcoplasm– Muscle enzyme systems become more

efficient because heat is increased as muscle contracts

Treppe